High-Resolution Initialization and Simulations of Typhoon Morakot (2009)

Abstract

A model self-bogus vortex is constructed by cycle runs using the Weather Research and Forecasting (WRF) model to provide high-resolution initial conditions for tropical cyclone (TC) simulations. The vortex after 1 h of model simulation is used to construct the vortex structure for the initial conditions for the next cycle run. After about 80 cycle runs, the TC structure is well adapted to the model employed and well adjusted to the given large-scale conditions. Three separate simulations using three different initial conditions including global analysis (CTRL), the bogus package from WRF (WB), and the new initialization package (NT) are performed for Typhoon Morakot (2009). The NT scheme shows advantages in generating realistic vortex features including sea level pressure, winds, a warm core, and correct TC size with the meteorological fields away from the observed TC center consistent with the global analysis. The NT scheme also shows significant improvements in TC simulations including asymmetric structure, track, intensity, strength of low-level winds, radar reflectivity, and rainfall. For other runs, such as WB and CTRL, the unbalanced initial vortex needs to adjust to the changing environment during the first 2–3 days of model simulations, which is likely to have negative impacts on the track, intensity, and rainfall forecasts in most cases. For all three different types of model initializations, the model is capable of simulating heavy orographic precipitation over southern Taiwan. However, with a better track forecast, only the NT run simulates the high-reflectivity band associated with the convergence zone between Morakot’s circulations and the southwest monsoon off the southeast coast. In addition to Morakot’s slow movement and relatively large size, Typhoons Goni and Etau were embedded within a moist monsoon gyre. The combined circulations associated with the monsoon gyre and tropical storms bring in moisture-laden flows toward the western slopes of southern Taiwan.